Apparatus for optical applications, spectrometer system and method for producing an apparatus for optical applications

ABSTRACT

The present invention relates to an apparatus for optical applications, a spectrometer system and method for producing an apparatus for optical applications, and in particular to an apparatus comprising an optical waveguide having a first refractive index along a light propagation axis interrupted by a plurality of scattering portions having a second refractive index. Each scattering portion has a long axis substantially perpendicular to the light propagation axis as well as a short axis substantially perpendicular to the light propagation axis and the long axis. A receiver unit or a transmitter unit is arranged on a side of the optical waveguide, the long axis being substantially perpendicular, i.e. normal to the plane of this side on which the receiver unit or transmitter unit is arranged. Accordingly, simplification and miniaturization of an optical apparatus can be realized.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/188,816, filed Mar. 1, 2021, which claims the benefit under 35 U.S.C.§ 120 as a continuation of U.S. application Ser. No. 16/486,987, filedAug. 19, 2019, which is a national stage filing under 35 U.S.C. § 371 ofInternational Patent Application Serial No. PCT/EP2018/054170, filedFeb. 20, 2018, which claims priority to German application number 102017 202 760.6, filed Feb. 21, 2017. The entire contents of theseapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to an apparatus for optical applications,a spectrometer system and method for producing an apparatus for opticalapplications, and in particular to an apparatus comprising an opticalwaveguide having a first refractive index along a light propagation axisinterrupted by a plurality of portions having a second refractive index.

BACKGROUND

Optical Bragg gratings, such as a Fiber Bragg Grating (FBG), are wellknown. A FBG is a type of distributed Bragg reflector in an opticalfiber that reflects selected wavelengths of light and transmits others.The Bragg grating is constituted by a periodic variation in therefractive index of the fiber core generating a wavelength-specificdielectric mirror due to selected interferences. A FBG can thus act asan optical filter to block/reflect certain wavelengths. The Bragggrating formed in a fiber occupies commonly a small segment of the fiberhaving a length in the range of one millimeter to several centimeters.

Optical Bragg gratings may also be used to couple out light of aspecific wavelength from the waveguide, wherein in such gratings thevariation of the refractive index along the optical axis of thewaveguide, for example an optical fiber, is not uniform across the widthof the fiber but the variation of the refractive index is at an anglebetween the optical axis and an axis perpendicular thereto. Thus, suchgratings are called tilted fiber Bragg gratings. The angle of tilt hasto be chosen sufficiently large so that the light with a wavelengthfulfilling the Bragg condition can escape the waveguide. Further, thereflection is polarization dependent so that only the light linearlypolarized in the plane of the grating is reflected. Accordingly, such agrating is wavelength and polarization selective.

Dispersive scattering of light on gratings is also known but usuallyundesired, since it leads to an attenuation of the light travelingthrough the waveguide. Particularly strong losses due to diffractivescattering can be observed for Type II gratings, written by highintensity UV lasers or femtosecond lasers, both operating above thedamage threshold of glass. For example, a femtosecond laser may be usedto write a grating point-by-point with pulse energy of approximately 0.1μJ or more and pulse duration of approximately 100 fs.

By studying the scattering of gratings in more detail the inventor cameto the conclusion that the usually undesired scattering effect could beused for optical applications by engineering the grating according tothe needs so that simplification and miniaturization of opticalapparatuses can be realized.

SUMMARY

Therefore, it is an aim to provide a novel apparatus for opticalapplications using a waveguide as well as a method for producing thesame.

According to an embodiment, an apparatus for optical applicationscomprises an optical waveguide configured to guide light along a lightpropagation axis and having a first refractive index along the lightpropagation axis interrupted by a plurality of portions having a secondrefractive index, wherein each portion has a long axis substantiallyperpendicular to the light propagation axis as well as a short axissubstantially perpendicular to the light propagation axis and the longaxis. Further, the apparatus comprises a receiver unit arranged on aside of the optical waveguide. The receiver unit is arranged so as toreceive light scattered from the plurality of portions in a scatteringdirection lying in a main scattering plane defined by the long axis andthe light propagation axis. Accordingly, an apparatus may be providedwhich allows to efficiently couple out light from an optical waveguideand to receive the light at the side of the waveguide at a particularangle with respect to the light propagation axis of the waveguide.

According to another embodiment, an apparatus for optical applicationscomprises an optical waveguide configured to guide light along a lightpropagation axis and having a first refractive index along the lightpropagation axis interrupted by a plurality of portions having a secondrefractive index, wherein each portion has a long axis substantiallyperpendicular to the light propagation axis as well as a short axissubstantially perpendicular to the light propagation axis and the longaxis. Further, the apparatus comprises a transmitter unit arranged on aside of the optical waveguide. The transmitter unit is arranged so as totransmit light to the plurality of portions in a scattering directionlying in a main scattering plane defined by the long axis and the lightpropagation axis, wherein the scattering direction has a scatteringangle with respect to the light propagation axis. Accordingly, anapparatus may be provided which allows to efficiently couple light in anoptical waveguide by sending the light onto the waveguide, in particularonto the segment including the plurality of portions having the secondrefractive index, at a particular angle with respect to the lightpropagation axis of the waveguide.

According to an advantageous example, the receiver unit of the apparatusmay be a detector comprising detector elements arranged in a line fordetecting the light scattered from the plurality of portions in thescattering direction. In particular, light of different wavelengths mayhave different scattering directions and thus different angles withrespect to the light propagation axis of the waveguide so that differentdetector elements detect different wavelengths of light. Therefore,dependent on the detector element detecting light, a wavelength can bedetermined.

According to another advantageous example, the line of the detectorelements is substantially parallel to the light propagation axis andlies in the main scattering plane. Accordingly, light scattered from theplurality of scattering portions of the optical waveguide can bedetected with high efficiency outside of the optical waveguide.

According to another advantageous example, the plurality of portionsforms a diffraction grating in the direction of the light propagationaxis. In one example, the distances between the portions are selected sothat in the main scattering plane a lens function is obtained byinterference of the light scattered at the plurality of portions. In analternative or additional example, the light propagation axis of thewaveguide is bent so that in the main scattering plane a lens functionis obtained by interference of the light scattered at the plurality ofportions arranged along the bent light propagation axis. Accordingly,the segment having the plurality of portions forming a grating is notlimited to periodically arranged portions but the distances between theportions can be chosen and engineered in such a way that scattering ordiffraction on the portions leads to interference effects whichcorrespond to optical lens functions.

In one example, the lens function corresponds to a focusing lensfocusing light scattering at the plurality of portions onto the receiverunit or focusing light from the transmitter unit into the opticalwaveguide by scattering at the plurality of portions. Accordingly,additional optics or optics external to the waveguide can be dispensedwith so at to simply the apparatus for optical applications, such asspectrometry.

According to another advantageous example, each portion of the opticalwaveguide having the second refractive index has a shape approximatingan ellipsoid, wherein the long axis of the ellipsoid is larger thantwice the wavelength of the light guided in the optical waveguide andthe short axis of the ellipsoid is in the order of or smaller than thewavelength of the light. Accordingly, much more light is scattered in amain scattering plane defined by the long axis and the light propagationaxis than in a minor scattering plane defined by the short axis and thelight propagation axis increasing the efficiency when light is detectedin the main scattering plane, for example.

According to another advantageous example, the optical waveguide furthercomprises an optical Bragg grating along the light propagation axis soas to provide diffracted light. Accordingly, a (normal) Bragg gratingmay be included in the waveguide which provides the diffracted referencelight, for example, light of a particular wavelength back-reflected inthe direction of the light propagation axis.

According to another advantageous example, the plurality of portions areadapted so that the scattering direction in the main scattering planefor light propagating in one propagation direction of the lightpropagation axis is different to another scattering direction in themain scattering plane for light propagating in the opposite direction ofthe propagation direction. Accordingly, light of different, e.g.opposing, propagation directions is coupled out in different scatteringdirections so that it is simple to separate light from differentpropagation directions spatially outside the waveguide.

According to another embodiment, a spectrometer system is providedcomprising a light source and the above-mentioned apparatus, wherein thelight of the light source is coupled into the optical waveguide at aninput port at the front face of the waveguide so as to guide the lightalong the light propagation axis. Accordingly, a spectrometer having asmall size can be realized.

According to another advantageous example, the spectrometer systemcomprises an optical Bragg grating sensor coupled to an output port ofthe optical waveguide so as to receive light propagating along the lightpropagation axis.

Accordingly, light affected by the Bragg grating sensor can be easilyanalyzed with a simple and small spectrometer system.

According to another advantageous example, the optical waveguidecomprises at least two light propagation axes being substantiallyparallel to each other. Accordingly, light coupled in the opticalwaveguide can be analyzed or processed differently depending on itslight propagation axis.

According to another advantageous example, the spectrometer systemcomprises at least two of the above-mentioned apparatuses arranged oneafter the other in the direction of the light propagation axis.Accordingly, light coupled in the optical waveguide can be analyzed orprocessed sequentially when propagating through the optical waveguideand passing the first and subsequently the second apparatus.

According to another embodiment, a method for producing an apparatus foroptical applications comprises the step of providing an opticalwaveguide configured to guide light along a light propagation axis andhaving a first refractive index along the light propagation axis. Themethod further comprises the step of focusing short laser pulses on thelight propagation axis to produce a plurality of portions having asecond refractive index so that each portion has a long axissubstantially perpendicular to the light propagation axis and a shortaxis substantially perpendicular to the light propagation axis and thelong axis.

Additionally, the method comprises the step of arranging a receiver uniton a side of the optical waveguide so as to enable the reception oflight from the plurality of portions in a scattering direction lying ina main scattering plane defined by the long axis and the lightpropagation axis. Alternatively, the method comprises the step ofarranging a transmitter unit on a side of the optical waveguide so as toenable the transmission of light to the plurality of portions in ascattering direction lying in a main scattering plane defined by thelong axis and the light propagation axis. Accordingly, an apparatus isproduced which allows to efficiently couple out/in light from/to anoptical waveguide and to receive/transmit the light at/from the side ofthe waveguide at a particular angle with respect to the lightpropagation axis of the optical waveguide.

Further advantageous features of the invention are disclosed in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates elements of an apparatus for optical applicationsaccording to an embodiment of the invention.

FIG. 1B illustrates elements of an apparatus for optical applicationsaccording to another embodiment of the invention.

FIG. 2 illustrates in a cross-sectional view an exemplary waveguideusable for the apparatuses of FIGS. 1A and 1B.

FIG. 3 illustrates elements of a specific apparatus having an opticalfocusing function according to another embodiment of the invention.

FIG. 4 illustrates elements of a specific apparatus having an opticalfocusing function by bending a waveguide according to another embodimentof the invention.

FIG. 5 illustrates qualitatively the performance of a spectrometeraccording to one embodiment and a spectrometer of the prior art.

FIG. 6 illustrates elements of a specific apparatus having an opticalfocusing function for light in one propagation direction.

FIG. 7 illustrates the effects of the apparatus of FIG. 6 qualitativelyfor different wavelengths.

FIG. 8 illustrates elements of a specific apparatus having two lightpropagation axes.

FIG. 9 illustrates a spectrometer system according to an embodiment.

FIG. 10 illustrates a multicore interrogator system according to anotherembodiment.

FIG. 11 illustrates a simulation with spherical waves using principlesof geometrical optics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments are described with reference to the figures. It isnoted that the following description contains examples only and shouldnot be construed as limiting the invention.

In the following, similar or same reference signs indicate similar orsame elements or operations.

Embodiments generally relate to an apparatus for optical applications,such as spectroscopy or optical interrogation methods, comprising anoptical waveguide configured to guide light along a light propagationaxis and having a first refractive index along the light propagationaxis interrupted by a plurality of scattering portions having a secondrefractive index distributed over a segment of the optical waveguide.Each scattering portion has a long axis substantially perpendicular tothe light propagation axis as well as a short axis substantiallyperpendicular to the light propagation axis and the long axis. Areceiver unit or a transmitter unit is arranged on a side of the opticalwaveguide, the long axis being substantially perpendicular, i.e. normalto the plane of this side on which the receiver unit or transmitter unitis arranged.

The receiver unit is arranged so as to receive light scattered from theplurality of portions in a scattering direction lying in a mainscattering plane defined by the long axis and the light propagationaxis. In the alternative transmitter case, the transmitter unit isarranged so as to transmit light to the plurality of portions in ascattering direction lying in a main scattering plane defined by thelong axis and the light propagation axis.

That is, in the embodiments, a grating like structure having refractiveindex altered portions is introduced in a segment of a waveguide. Theportions are arranged one after the other having a material with anotherrefractive index in between so that light is scattered due to therefractive index differences between the refractive index of theportions and of the light guiding structure in the optical waveguide,wherein the scattered light interferes constructively at certain angleswith respect to the light propagation axis of the optical waveguide.Thus, light can be coupled out of the waveguide at the scatteringportions and similarly coupled into the waveguide at these portions.

FIG. 1A illustrates elements of an apparatus 100 according to anembodiment of the invention, comprising an optical waveguide, e.g. madeof a first layer 110, a second layer 120 and a third layer 115, as wellas a receiver unit 140, e.g. a detector. The receiver unit 140 it notlimited to a detector but in other examples a receiver unit 140 may beanother plurality of portions in another or even same waveguidereceiving the light and guiding it further.

Several examples of optical waveguides are known, e.g. optical fibers,such as glass fibers, polymer fibers, or a bulk-glass substrate withwaveguides, polymers with waveguide structures, etc., and the inventionis not limited to a particular waveguide. For example, an opticalwaveguide may be a dielectric slap waveguide having three dielectriclayers with different refractive indices, wherein the refractive indicesare chosen so as to guide light in the second (middle) dielectric layer.A common example which will be referred to in the following purely as anillustrative example is an optical fiber in which the layer 120 isregarded as the core surrounded by the cladding layer 110, 115. Thus, inthe example of the substantially cylindrical optical fiber, layers 110and 115 are the same belonging to the same cladding. A preferredembodiment of a fiber as an exemplary optical waveguide will bediscussed in more detail later with respect to FIG. 2 .

The optical waveguide 110, 115, 120 in FIG. 1A, which will be consideredto constitute an optical fiber to simplify the discussion in thefollowing, is configured to guide light along a light propagation axiswhich may be achieved by a core 120 having a first refractive index anda cladding 110 (same as 115) having a different refractive index andcoaxially surrounding the core. The core 120 usually guides the largestparts of the light intensity so that the center of the core of theoptical fiber may thus be considered to basically determine thedirection of the light propagation axis.

In the apparatus 100, the core 120 is interrupted several times by aplurality of portions having a second refractive index different fromthe first refractive index so that light diffracts/scatters at theportions. The portions 130 may be arranged periodically having all thesame distances from each other or the distances between the portions mayvary, for example, to achieve a specific optical function of the lightscattered from the portions and interfering constructively. When theterm light is mentioned herein, light is not limited to visible lightbut the herein described technology is also applicable to ultraviolet(UV) and infrared (IR) light.

A scattering portion 130 has a long axis substantially perpendicular tothe light propagation axis as well as a short axis substantiallyperpendicular to the light propagation axis and the long axis. Forexample, the scattering portion 130 may have the shape of an ellipsoid.These differences in the dimensions of the long and short axes of ascattering portion lead to large differences in the intensity of thescattered light. That is, more light is emitted from the waveguide inthe scattering direction lying in a main scattering plane defined by thelong axis and the light propagation axis than in a scattering directionlying in a minor scattering plane defined by the short axis and thelight propagation axis.

For example, to effect strong scattering/diffraction from the opticalwaveguide in a main scattering plane in FIG. 1A, the long axis may belarger than twice the wavelength of the light guided in the opticalwaveguide and the short axis may be in the order of or smaller than thewavelength of the light. Preferably, the distances between the portionsand the dimensions of the portions are selected so that most light isemitted in the light scattering direction in main scattering planehaving an angle between 30 and 150 degrees from the light propagationaxis. For example, the light scattering direction may overlap with adiffraction order, preferably the first diffraction order.

In FIG. 1A, the apparatus 100 comprises a receiver unit 140 realized asa detector in this example. This detector detects light scattered fromthe scattering portions 130. To detect the scattered light, the receiverunit is arranged on a side of the optical waveguide so that it receivesthe most of the scattered light, namely on a side in the main scatteringplane. The long axis being substantially perpendicular, i.e. normal, tothe plane of the side on which the receiver unit is arranged.

As can be seen in FIG. 1A, three different light scattering directionsin the main scattering plane (here the plane of the paper sheet) areshown for different wavelengths; the dashed line representing bluelight, the dotted line representing green light and the solid linerepresenting red light.

In one embodiment, the detector 140 of the apparatus 100 comprisesdetector elements arranged in a line for detecting the light scatteredfrom the plurality of portions in the scattering direction(s). Detectorelements are indicated on detector 340 of FIG. 3 which will be discussedlater. For example, in FIG. 1A, detector elements on the right side ofthe detector in FIG. 1A may detect the blue light, detector elements inthe middle may detect green light and detector elements on the left sidemay detect red light. Examples of a detector having detector elementsare a CCD chip, linear diode array, or similar.

In the example of FIG. 1A, the line of the detector elements issubstantially parallel to the light propagation axis and lies in themain scattering plane. In particular, the detector 140 is positioned onthe right side from the fiber core if a light propagating direction isconsidered from the left to right side of the paper.

It should be understood that the detector may be as well placed on theleft side from the fiber core (when seen in the light propagationdirection indicated by the arrow) in the same plane. However, althoughthe light scattered and diffracted from portions 130 is emitted in asymmetrical cone shape where the symmetry axis of the cone is the fibercore, positioning the detector above or below (the paper plane) may bedisadvantageous, since then only light scattered in a minor scatteringplane defined by the short axis and the light propagation axis can bereceived.

In FIG. 1B, apparatus 100′ is illustrated which comprises the sameoptical waveguide as apparatus 100 but with the difference that receiverunit 140 is replaced by transmitter unit 150. As explained above, theoptical waveguide 110, 120 may be an optical fiber having a fiber core120 and the cladding 110. The portions 130 again scatter and diffractlight but in FIG. 1B, the light is coupled into the core 120 of theoptical fiber, since it comes from the transmitter unit outside thewaveguide. A single wavelength is illustrated by a dashed line emittedby a laser which is an example of a transmitter unit 150.

The transmitter unit 150 is also arranged on a side of the opticalwaveguide. In particular, the transmitter unit 150 is arranged so as totransmit light from the outside of the optical waveguide to theplurality of portions in a scattering direction lying in the mainscattering plane defined by the long axis and the light propagationaxis. As can be seen in FIG. 1B, the scattering direction has ascattering angle of approximately 60° with respect to the lightpropagation axis.

In the apparatuses 100 and 100′ light scatters locally on the refractiveindex alterations introduced by the scattering portions. According togeometrical optics, the superposition of spherical waves generated bythe scattering portions may lead to the observed directional andwavelengths-dependent constructive interferences in certain angles withrespect to the light propagation direction (see FIG. 11 ). Bymanipulating the distances between the portions basically correspondingto the “lines” of a grating, the optical characteristics of theinterferences can be engineered.

The portions 130 act similarly to lines of a grating, however, theportions are not lines but preferably elliptical structures, such as anellipsoid which leads to more predominant scattering in the plane of thelong axis of the ellipsoid and the light propagation axis. For differentpolarization states a change in intensity of the stray light may beobserved whereas for spherical portions scattering appears polarizationindependent. Interestingly, the above-mentioned effects can be used forcoupling in or coupling out light in a particular scattering direction,as shown in FIGS. 1A and 1B.

FIG. 2 illustrates a cross-sectional view of an exemplary waveguide 200comprising a core 220 and a cladding 210. As explained above, in thelight propagation direction, the core having a first refractive index,such as 1.460 is interrupted by portions having a second refractiveindex, such as 1.462. The cross-section shown in FIG. 2 shows a portion230 interrupting the core, i.e. placed in the core so that lightpropagating to the core experiences a refractive index change, inparticular a larger effect is experienced in the long axis correspondingto the vertical axis in the FIG. than in the short axis corresponding tothe horizontal axis in the figure.

In FIG. 2 , the portion 230 that is one of the plurality of portions hasa shape approximating an ellipsoid in three dimensions (an ellipse isillustrated in two-dimensional FIG. 2 ), wherein the long axis of theellipsoid is preferably larger than twice the wavelength of the lightguided in the optical waveguide and the short axis of the ellipsoid isin the order of (i.e. 1×wavelength+/−0.5×wavelengths) or smaller thanthe wavelength of the light. According to one preferred example, thelong axis may be three times the wavelength or larger and the short axismay be 1.5 times the wavelength or smaller.

In FIG. 3 , apparatus 300 is illustrated which comprises basically thesame optical waveguide as apparatuses 100 and 100′ but the segment ofthe waveguide including the plurality of portions has been specificallyengineered to incorporate an optical function. In more detail, thedistances between the portions are selected in such a way that lightentering the waveguide on the left side indicated by the arrow andpropagating along the light propagation axis is scattered at thescattering portions so that the light of a specific wavelength isfocused on a particular point on the side of the waveguide, preferablyon a detector, such as detector 340.

Since different wavelengths are scattered differently and have differentscattering directions (and scattering angles) in which they interfereconstructively, different wavelengths can be focused on different partsof the detector 340. In particular, the solid line representing bluelight focuses on the right side of the detector, the dashed linerepresenting green light focuses on the middle of the detector and thedotted line representing red light focuses on the left side of thedetector.

In the example of FIG. 3 , the plurality of portions forms a diffractiongrating in the core 120 of the waveguide in the direction of the lightpropagation axis. In particular, in this embodiment, the grating alsoacts as imaging optics by focusing light. Surely, the optical functioncoded into the plurality of portions by a particular selection of thedistances is not limited to a focusing lens function.

In other words, the distances between the portions are selected so thatin the main scattering plane a lens function is obtained by interferenceof the light scattered at the plurality of portions 330, wherein in oneexample the lens function corresponds to a focusing lens focusing lightscattering at the plurality of portions. Accordingly, direct imaging oflight on a detector is possible, since an aspheric imaging function canbe integrated in the waveguide by specific distances between the gratingportions.

Instead of selecting specific distances between the portions whenmanufacturing the waveguide as in FIG. 3 , a similar effect may beachieved by bending an optical fiber having scattering portions arrangedperiodically. This basically corresponds to bending the lightpropagation axis so that in the main scattering plane a lens function isobtained by interference of the light scattered at the plurality ofportions arranged along the bent light propagation axis. An example ofproviding the optical function of a focusing lens is provided in FIG. 4.

In FIG. 4 , apparatus 400 comprises a bend waveguide, wherein thecurvature leads to constructive interference in the same way as in FIG.3 so as to obtain an effect of focusing different wavelengths on adetector.

In one embodiment, the apparatuses 300 and 400 may be used as aspectrometer in which a special slit, grating or imaging optics are notneeded, since the waveguide and its light guiding section act as slitand the plurality of scattering portions acts as diffraction gratingwhich largely simplifies the structure of the spectrometer.

According to one embodiment, a spectrometer system comprising a lightsource and one of the above mentioned apparatuses 100, 100′, 300 or 400is provided, wherein the light of the light source is coupled into theoptical waveguide at an input port so as to guide the light along thelight propagation axis. The light source may be preferably a lightsource with a broad wavelength range, such as a super luminescent diode(S-LED) or white light from a halogen or tungsten lamp.

As a result of the above, to obtain a spectrometer merely two parts haveto be manufactured and arranged with respect to each other, namely theabove-mentioned waveguide and a detector. More importantly, focuslength, size of the spectrometer and the wavelength range to be analyzedare freely selectable leading to much simpler and smaller spectrometersthan in the prior art that require complicated focusing optics andexpensive line gratings.

Since the positional relationship between the focusing optics and theline grating in the prior art is crucial, the here proposed spectrometerin which the focusing optics is integrated in the grating is more rigidand misalignment can hardly occur. Due to the possibility of a largespectral separation of wavelengths and integration of optics in thewaveguide, the herein described spectrometer can be more than five timessmaller than existing spectrometers having the same properties butoptics arranged in free space. Further, the herein describedspectrometer may have a large number of portions (more than 10000) alongthe light propagation axis so that the spectral resolution can be easilyincreased.

FIG. 5 illustrates qualitatively the performance of a spectrometeraccording to one embodiment (b) and a spectrometer of the prior art (a).To compare spectra from a conventional spectrometer with a spectrometeraccording to the inventive concept, a 2×2 fiber coupler has been used,wherein the upper input port of the coupler was connected to an S-LEDand the upper output port to an FBG sensor fiber producing 15 peaks. Theback reflected signal of the sensor was then measured at the loweroutput port of the fiber coupler in (a) with an OCEAN-Optics Flamespectrometer which was replaced in (b) by the apparatus 300 having as adetector a commercial WebCam module at 2 cm focal length and standarddrivers of MS Windows.

The spectra in FIG. 5 show arbitrary units on the Y axis and wavelengthon the X axis. It can be seen that qualitatively the ability ofseparating wavelengths is basically the same for the expensive prior artspectrometer resulting in the spectrum a) on the left side of FIG. 5 andfor the low-cost spectrometer of the embodiment resulting in thespectrum b) on the right side.

One of the problems in a known interrogator system having a spectrometerbased Bragg grating is that such a system requires at least one opticalcirculator or coupler to direct the light reflected back from a FBGsensor to a detector. For systems having a low light level, e.g. whenanalyzing an FBG in a fiber using an LED, intensity losses of the lightsource coupled to a fiber and passing a 2×2 coupler are large and thusreduction thereof is important. Further, costs are large because acoupling element is needed which further contributes to the size of thesystem. An apparatus having a waveguide and a plurality of speciallyarranged portions, as described above, may mitigate some of theseproblems.

FIG. 6 illustrates elements of a specific apparatus 600 having anoptical focusing function for light in one propagation direction (seesolid line) realized by scattering portions 630 similar to apparatus 300of FIG. 3 and having diffuse scattering in the other direction (seedashed line). The diffuse scattered light can then be trapped by a lighttrap 650. That is, the angles of the light scattering directions arelargely different for the two propagation directions in FIG. 6 so thatthe scattered light can be detected in largely different spatialdirections and positions.

In this embodiment, the plurality of portions are adapted so that thescattering direction in the main scattering plane for light propagatingin one propagation direction of the light propagation axis is differentto another scattering direction in the main scattering plane for lightpropagating in the opposite direction of the propagation direction.Since the scattering directions are different, light of one propagationdirection falls on the detector 340 in a focused fashion and the lightof the opposite propagation direction will not. In essence, bymodification of the grating period, symmetry is broken and theabove-mentioned effect can be achieved.

Furthermore, a part of the light in the waveguide scatters at thegrating according to the efficiency of the grating and another partwhich can be determined by the process parameters is simply transmittedto reach an FBG sensor where it is reflected (see e.g. FIGS. 9 and 10 ).The reflected light can then be focused on the detector 340 on the wayback.

For example, by using the above mentioned diffraction grating with animaging function incorporated in a waveguide, a known interrogatorsystem having a spectrometer based Bragg grating can be greatlysimplified. Accordingly, a more cost efficient and smaller interrogatorsystem can be realized which requires merely a light source, an opticalwaveguide and a detector, such as a camera, linear diode array orindividual photodiodes. Most of the functions can be realized in thewaveguide, e.g. fiber, so that there is not much space needed for otheroptics or fiber windings. Such an interrogator can thus be directlyintegrated in measurement devices which have limited space.

FIG. 7 illustrates the effects of the apparatus 600 of FIG. 6qualitatively for different wavelengths. In the first direction fromleft to right, it is simulated how red (r), green (g) and blue (b) lightis scattered at portions in a fiber segment and focused on the line 710.In the opposite direction from the right to left, the simulation in FIG.7 shows that there is no focusing effect and light is scattered in acompletely different direction from the light in the first direction.

FIG. 9 illustrates a spectrometer system 900 according to an embodiment.The spectrometer system 900 is basically an interrogator system andcomprises the apparatus 600 including a detector 640 and a light trap650 for diffusively scattered light. The spectrometer system 900comprises an optical Bragg grating acting as a sensor sensing physicalparameters, such as temperature, at an object 980. The optical Bragggrating sensor is coupled to an output port of the optical waveguide soas to receive light propagating along the light propagation axis.Alternatively, the optical Bragg grating sensor is integrallyincorporated in the optical waveguide or fiber of apparatus 600.

In operation, light from the light source, e.g. S-LED, propagating fromleft to right is scattered at the plurality of portions and thescattered light is received in the light trap 650. The unscattered partof the light is further transmitted to the fiber Bragg grating at theobject to be measured and then back reflected at the FBG and scatteredagain at the plurality of portions, wherein in the direction from rightto left the scattered light is now focused on the detector 640. Thus,the system 900 comprises merely three elements and has a much higherlight efficiency than a system with an optical circulator or coupler.

Another application of the inventive concept is to multicore fibers.Conventionally, when using multicore fibers, if the propagating lightneeds to be analyzed, each core of the multicore fiber needs to becoupled to individual single-core fibers. To do this, a so-calledfan-out device is known which is complicated to manufacture andexpensive as well as difficult to align. When using the above principleto output/input light propagating in a fiber by a plurality ofscattering portions, it is not necessary to couple individual fibers tothe multicore fiber.

FIG. 8 illustrates elements of a specific apparatus 800 having two lightpropagation axes. In particular, light propagates in a first core 120parallel to a second core 125 in the same waveguide. In both cores aplurality of scattering portions 830 and 835, respectively, are providedwhich focus light on detector 140. In essence, two apparatuses 300 or600 are used.

FIG. 10 illustrates a multicore interrogator system 1000 according toanother embodiment. In this embodiment, the optical waveguide comprisesat least two light propagation axes being substantially parallel to eachother, e.g. at least two fiber cores in a multicore fiber. The light ofthe different light propagation axes of the different cores can then beanalyzed core by core by outputting the light on a long detector orseveral detectors, as indicated in FIG. 10 .

In more detail, the multicore interrogator system 1000 comprises anS-LED as light source and an FBG as sensor at an object 1080. Further,it comprises four cores, namely core1, core 2, core3 and core4. Eachcore and its associated detector and light trap constitutes basically anapparatus 600. Accordingly, a multicore interrogator system can berealized without using a coupling element or additional opto-mechaniccomponents.

Further, spectrometer based FBG interrogator systems often show atemperature-dependent signal drift in a warming up phase. When thespectrometer warms up changes in the optical path, for example expansionof optics or grating, may occur so as to change the detectedwavelengths. Instead of providing a complicated temperature adjustmentmethod, a bidirectional fiber optic spectrometer, such as the system 900in FIG. 9 , may be provided. That is, a conventional Bragg grating maybe added to or incorporated in the fiber optic spectrometer. This Bragggrating may then act as a reference light source for easy temperaturecompensation.

Still further, by using the above described scattering effects, weaklyreflecting and/or homogeneous Bragg gratings of different lengths may beincorporated in a waveguide so that light coupled out from the waveguidecan be directly read by a conventional barcode reader, e.g. foridentifying the waveguide. If the grating(s) is/are weakly reflecting,there is hardly any effect on the sensor signal. In this way, anunambiguous association of the fiber, e.g. by identifying place and dateof manufacture, can be realized.

FIG. 11 illustrates a simulation of the scattering effects explained inthe above embodiments using spherical waves and the principles ofgeometrical optics. According to geometrical optics, the superpositionof spherical waves generated by the scattering portions, indicated asstars (*) in FIG. 11 , may lead to the observed directional andwavelengths-dependent constructive interferences in certain angles withrespect to the light propagation axis (the horizontal axis in FIG. 11 ).

In general, diffractive scattering occurs over the length of the gratingand at least one diffraction order having a particular diffraction anglecan be observed which can be approximated by the Bragg equation fordiffraction. Thus, in a waveguide, such as an optical fiber, which has acylindrical symmetry, light is emitted in the shape of a cone around thelight propagation axis of the fiber as a symmetry axis with an openingangle corresponding to the diffraction angle (approximately 80° in FIG.11 ).

For example, the scattering portions forming a grating may be producedlike the above-mentioned type II gratings, written by high intensityfemtosecond lasers, operating above the damage threshold of glass. Forexample, a femtosecond laser may be used to write a gratingpoint-by-point with pulse energy of approximately 0.1 μJ or more andpulse duration of 500 fs or shorter.

The scattering and/or diffraction patterns observed on a type II gratingmay be explained like this. When producing a scattering portion of thegrating with a femtosecond laser, this portion is not homogenous andseveral microscopic structures of different sizes in the scatteringportions of the grating lead to different scattering and diffractioneffects. Accordingly, the refractive index of a scattering portion maybe regarded as an average refractive index resulting from severaldifferent microscopic defects resulting from the pulsed laser pulsesdestroying the homogenous structure of the core of the waveguide.

According to another embodiment, a method for producing an apparatus foroptical applications comprises providing an optical waveguide configuredto guide light along a light propagation axis and having a firstrefractive index along the light propagation axis and then focusingshort laser pulses on the light propagation axis to produce theplurality of portions having a second refractive index so that eachportion has a long axis substantially perpendicular to the lightpropagation axis and a short axis substantially perpendicular to thelight propagation axis and the long axis. Further, the method comprisesarranging a receiver unit or transmitter unit on a side of the opticalwaveguide so as to enable the reception or transmission of light from orto the plurality of portions in a scattering direction lying in a mainscattering plane defined by the long axis and the light propagationaxis.

As described above, embodiments and examples of the invention allow forcoupling in and coupling out light. Therefore, a simple and small lightcoupling apparatus may be provided.

It will be appreciated that various modifications and variations can bemade in the described apparatuses, systems and methods as well as in theconstruction of this invention without departing from the scope orspirit of the invention.

The invention has been described in relation to particular embodimentsand examples which are intended in all aspects to be illustrative ratherthan restrictive.

Moreover, other implementations of the invention will be apparent to theskilled person from consideration of the specification and practice ofthe invention disclosed herein. It is intended that the specificationand the examples be considered as exemplary only. To this end, it is tobe understood that inventive aspects lie in less than all features ofthe foregoing disclosed implementation or configuration. Thus, the truescope and spirit of the invention is indicated by the following claims.

1. An apparatus for optical applications, comprising an opticalwaveguide configured to guide light along a light propagation axis andhaving a first refractive index along the light propagation axisinterrupted by a plurality of portions having a second refractive index,wherein each portion of the plurality of portions has a long axisperpendicular to the light propagation axis and a short axisperpendicular to the light propagation axis and the long axis; and areceiver arranged on a side of the optical waveguide, wherein thereceiver is arranged so as to receive light scattered from the pluralityof portions, wherein the optical waveguide further comprises an opticalBragg grating along the light propagation axis.
 2. The apparatus foroptical applications according to claim 1, wherein the receivercomprises optical detectors arranged in a line for detecting the lightscattered from the plurality of portions in the scattering direction. 3.The apparatus for optical applications according to claim 2, wherein theline of the optical detectors is parallel to the light propagation axis.4. The apparatus for optical applications according to claim 1, whereinthe plurality of portions forms a grating in the direction of the lightpropagation axis, and wherein the distances between the portions areselected so that in a main scattering plane a lens function is obtainedby interference of the light scattered at the plurality of portions, orwherein the light propagation axis is bent so that in a main scatteringplane a lens function is obtained by interference of the light scatteredat the plurality of portions arranged along the bent light propagationaxis.
 5. The apparatus for optical applications according to claim 4,wherein the lens function corresponds to a focusing lens focusing lightscattered at the plurality of portions.
 6. The apparatus for opticalapplications according to claim 1, each portion of the plurality ofportions having an ellipsoid shape, wherein a length of the long axis ofthe ellipsoid shape is larger than twice a wavelength of the lightguided in the optical waveguide and a length of the short axis of theellipsoid shape is in an order of or smaller than the wavelength of thelight.
 7. A spectrometer system, comprising the apparatus according toclaim
 1. 8. The spectrometer system according to claim 7, furthercomprising a detector, wherein the optical Bragg grating acts as asensor sensing physical parameters, such as temperature.
 9. Thespectrometer system according to claim 8, wherein the optical waveguidecomprises at least two light propagation axes being parallel to eachother.
 10. An apparatus for optical applications, comprising an opticalwaveguide configured to guide light along a light propagation axis andhaving a first refractive index along the light propagation axisinterrupted by a plurality of portions having a second refractive index,wherein each portion of the plurality of portions has a long axisperpendicular to the light propagation axis and a short axisperpendicular to the light propagation axis and the long axis; and atransmitter arranged on a side of the optical waveguide, wherein thetransmitter is arranged so as to transmit light to the plurality ofportions, wherein the optical waveguide further comprises an opticalBragg grating along the light propagation axis.
 11. The apparatus foroptical applications according to claim 10, wherein the plurality ofportions forms a grating in the direction of the light propagation axis,and wherein the distances between the portions are selected so that in amain scattering plane a lens function is obtained by interference of thelight scattered at the plurality of portions, or wherein the lightpropagation axis is bent so that in a main scattering plane a lensfunction is obtained by interference of the light scattered at theplurality of portions arranged along the bent light propagation axis.12. The apparatus for optical applications according to claim 11,wherein the lens function corresponds to a focusing lens focusing lightscattered at the plurality of portions.
 13. The apparatus for opticalapplications according to claim 10, each portion having an ellipsoidshape, wherein a length of the long axis of the ellipsoid shape islarger than twice a wavelength of the light guided in the opticalwaveguide and a length of the short axis of the ellipsoid shape is in anorder of or smaller than the wavelength of the light.
 14. A method forproducing an apparatus for optical applications, comprising providing anoptical waveguide configured to guide light along a light propagationaxis and having a first refractive index along the light propagationaxis, focusing short laser pulses on the light propagation axis toproduce a plurality of portions having a second refractive index so thateach portion of the plurality of portions has a long axis perpendicularto the light propagation axis and a short axis perpendicular to thelight propagation axis and the long axis; and arranging a receiver ortransmitter on a side of the optical waveguide so as to enable thereception or transmission of light from or to the plurality of portionsin a scattering direction lying in a main scattering plane defined bythe long axis and the light propagation axis, wherein the opticalwaveguide further comprises an optical Bragg grating along the lightpropagation axis acting as a reference for thermal compensation of theapparatus.
 15. A spectrometer comprising an apparatus for opticalapplications, the apparatus comprising an optical waveguide configuredto guide light along a light propagation axis and having a firstrefractive index along the light propagation axis interrupted by aplurality of portions having a second refractive index, wherein eachportion of the plurality of portions has a long axis perpendicular tothe light propagation axis as well as a short axis perpendicular to thelight propagation axis and the long axis; and a receiver arranged on aside of the optical waveguide, wherein the receiver is a detectorarranged so as to receive light scattered from the plurality of portionsin a scattering direction; the detector comprises optical detectorelements arranged in a line for detecting the light scattered from theplurality of portions in the scattering direction, wherein the distancesbetween the portions and the dimensions of the portions are selected sothat light entering the optical waveguide and propagating along thelight propagation axis is scattered at the plurality of portions so thatlight of a specific wavelength is focused on a particular point of thedetector on the side of the optical waveguide, wherein differentwavelengths are scattered differently and have different scatteringdirections in which they interfere constructively so that differentwavelengths are focused on different optical detector elements of thedetector.
 16. The spectrometer according to claim 15, each portion ofthe plurality of portions having an ellipsoid shape, wherein a length ofthe long axis of the ellipsoid shape is larger than twice a wavelengthof the light guided in the optical waveguide and a length of the shortaxis of the ellipsoid shape is in the order of or smaller than thewavelength of the light.
 17. The spectrometer according to claim 15,wherein the optical waveguide further comprises an optical Bragg gratingalong the light propagation axis.
 18. A method for producing anapparatus for optical applications, comprising providing an opticalwaveguide configured to guide light along a light propagation axis andhaving a first refractive index along the light propagation axis,focusing short laser pulses on the light propagation axis to produce aplurality of portions having a second refractive index so that eachportion has a long axis perpendicular to the light propagation axis anda short axis perpendicular to the light propagation axis and the longaxis; and arranging a receiver on a side of the optical waveguide so asto enable the reception of light from the plurality of portions in ascattering direction, the receiver being a detector comprising opticaldetector elements arranged in a line for detecting the light scatteredfrom the plurality of portions in the scattering direction, wherein thedistances between the portions and the dimensions of the portions areselected so that light entering the optical waveguide and propagatingalong the light propagation axis is scattered at the plurality ofportions so that light of a specific wavelength is focused on aparticular point of the detector on the side of the optical waveguide,wherein different wavelengths are scattered differently and havedifferent scattering directions in which they interfere constructivelyso that different wavelengths are focused on different optical detectorelements of the detector.